Many couples who use the rhythm method to prevent pregnancy end up pregnant because it is impossible to predict when a woman will release an egg.

The average woman is fertile for only 6 days out of her 28-day cycle. An egg can live only one day after it is released, while the longest sperm can live in the vagina is five days. A woman's cycle is dated from the first day of menstruation. An interesting study in this month's British Medical Journal shows that only about 30 percent of women release their eggs to make them fertile from day 10 to 17. Even in women with regular repeatable menstrual cycles, most women reach their fertile days earlier and many much later. Because of this extreme variability in egg release, most women need more than the rhythm method to prevent pregnancy.

AJ Wilcox, D Dunson, DD Baird. The timing of the ''fertile window'' in the menstrual cycle: day specific estimates from a prospective study. British Medical Journal, 2000, Vol 321, Iss 7271, pp 1259-1262 Address Wilcox AJ, NIEHS, Epidemiol Branch, Res Triangle Pk,NC 27709 USA

Checked 8/9/05

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